Polar ice consists of sea ice formed from the freezing of sea water, and ice sheets and glaciers formed from the accumulation and compaction of falling snow. Both types of ice extend over vast areas of the polar regions. Global sea-ice coverage averages approximately 25 million km2, the area of the North American continent, whereas ice sheets and glaciers cover approximately 15 million km2, roughly 10% of the Earth's land surface area.

Effects on Energy Exchange
Ice, both on land and in the sea, affects the exchange of energy continuously taking place at the Earth's surface. Ice and snow are among the most reflective of naturally occurring Earth surfaces. In particular, sea ice is much more reflective than the surrounding ocean, so that if it were to increase in extent, for instance because of large-scale cooling, then more solar energy would be reflected back to space and less would be absorbed at the surface. This would tend to cool the local region further, with the likelihood that more ice would be formed and still more cooling would occur.

On the other hand, if global warming occurs, then more ice would be expected to melt, reducing the energy reflected back to space and increasing the energy absorbed at the surface. The affected portions of the Earth would become still warmer. Scientists refer to this kind of reinforcing process as a "positive feedback."